In the field of miniaturized electronic circuits and integrated circuit packages, high-precision manufacturing and quality control are important. There is a need for accuracy in measurements and control of parameters for reducing defects in packaged semiconductor products, which will ultimately reduce costs associated with the process. Of particular concern is the coplanarity, or relative heights, of three-dimensional features such as solder balls in Ball Grid Array (BGA) devices or leads in Quad Flat Package (QFP) devices, which are critical to the performance of a circuit board. For example, if a solder ball has an insufficient height or is below the average height of other solder balls, there will not be any electrical connection between the solder ball and its corresponding pad on the circuit board, leading to a non-performing circuit board.
Several types of inspection apparatus have been proposed. For instance, U.S. Pat. No. 7,508,974 discloses a calibration and part inspection method for the inspection of BGA devices for measurement of heights of solder balls. Two cameras image a precision pattern mask with dot patterns deposited on a transparent reticle. The precision pattern mask is used for calibration of the system. A light source and overhead light reflective diffuser provide illumination, and a first camera images the reticle precision pattern mask from directly below. An additional mirror or prism located below the bottom plane of the reticle reflects the reticle pattern mask from a side view, through prisms or reflective surfaces, into a second camera, and a second additional mirror or prism located below the bottom plane of the reticle reflects the opposite side view of the reticle pattern mask through prisms or mirrors into the second camera. By imaging more than one dot pattern, the missing position values of the system can be resolved using a trigonometric solution. The reticle with pattern mask is removed after calibration, and the BGA device to be inspected is placed with the balls facing downward, in such a manner as to be imaged by the two cameras. The images of the BGA device are processed using a triangulation method to calculate a three-dimensional position of at least one ball with reference to a pre-calculated calibration plane.
U.S. Pat. No. 7,573,569 discloses an inspection system that combines 2-D inspection and 3-D inspection of the components of an electronic device into one compact module. The inspection system discloses a 2-D image acquisition assembly for inspecting 2-D criteria of the components, a 3-D image acquisition assembly for inspecting 3-D criteria of the components, and a computer for control and data analyzing. The 3-D image acquisition assembly comprises a 3-D image sensor and a light source. The 3-D light source is preferably a laser capable of generating a planar sheet of light that is substantially perpendicular to the inspection plane of the electronic device. The 2-D image acquisition assembly comprises a 2-D sensor and a 2-D light source positioned above the holder. The 2-D and 3-D image acquisition assemblies are arranged so that the 2-D inspection and 3-D inspection can be done while the electronic device is being held in one location.
EP Patent No. 0638801 B1 discloses a system to measure x, y, and z positional and size data of a BGA using a two camera “stereo” arrangement. One camera is used to measure the centrality of a BGA, and the other is used to sense the flatness. The flatness is measured by a tilted camera sensing crescent-shaped reflections from a light source located opposite the camera.
The above inventions suffer from limited depth of focus for the side view cameras. Some of the inspection apparatuses disclose systems made in accordance with the Scheimpflug principle. For many years, it has been known that under certain circumstances, as defined by the Scheimpflug principle, it is possible to simultaneously focus on several things at different distances from the camera, providing that all the objects of interest lie on a flat plane. Once this condition is satisfied, the object plane, the image plane, and a plane passing through the lens, all intersect along a line.
For example, U.S. Pat. No. 6,671,397 discloses a measurement system having a camera with a lens and a separate sensor mounted so that their planes intersect at an object plane in accordance with the Scheimpflug principle. A reference camera is normal and provides a two-dimensional image which is used by an image processor to determine a calibration image. This allows the image processor to determine the height of the relevant object. A single image capture provides an image of the full object, such as a BGA device. However, the image of the full object from the technique disclosed suffers from non-uniformity issues, resulting in a focused but non-uniform image, which hinders the accuracy of the results obtained.
It is, therefore, a motivation of the present invention to improve the accuracy of the measurements in vision inspection systems by producing well-focused images of uniform light intensity.
Any discussion of documents, devices, acts or knowledge in this specification is included to explain the context of the invention. It should not be taken as an admission that any of the material forms a part of the state of the art or the common general knowledge in the relevant art anywhere on or before the priority date of the disclosure and claims herein. All statements as to the date or representation as to the contents of these documents is based on the information available to applicants and does not constitute any admission as to the correctness of the date or contents of these documents.